The invention relates to acoustic transducers, and more particularly, to flextensional projectors having shell geometry allowing a substantially constant driver stress over a broad range of depths.
Acoustical transducers convert electrical energy to acoustical energy, and vice-versa, and can be employed in a number of applications. For example, transducers are a primary component used in sonar applications such as underwater seismic prospecting and detection of mobile vessels. In such applications, acoustic transducers are generally referred to as projectors and receivers. Projectors convert electrical energy into mechanical vibrations that imparts sonic energy into the water. Receivers are used to intercept reflected sonic energy and convert the associated mechanical vibrations into electrical signals. Multiple projectors and receivers can be employed to form arrays for detecting underwater objects.
In a typical underwater application, marine vessels tow acoustic projectors that generate acoustical energy in the surrounding area to conduct geophysical testing. The acoustical energy travels through the water and underlying subsurface geologic structures. Some of the acoustical energy is reflected back from the geologic structures and is detected with geophone or hydrophone sensors.
A projector typically includes an electromechanical stack of ceramic or rare earth elements having a particular crystalline structure. Depending on its crystal structure and material, a projector may be, for example, piezoelectric, electrostrictive, or magnetostrictive. For instance, if a ceramic crystal is subjected to a high direct current voltage during the manufacturing process, the ceramic crystal becomes permanently polarized and operates as a piezoelectric. An electrical signal applied to the ceramic crystal generates mechanical vibrations. A plurality of such crystals can be configured in a stack to provide greater vibrations, and is commonly referred to as a xe2x80x9cdriverxe2x80x9d or xe2x80x9ctransduction driver.xe2x80x9d
In another instance, direct current voltage can be temporarily applied to a ceramic stack during operation to provide polarization of the crystals. Under such conditions, the operation of the projector is electrostrictive. After the application of direct current voltage is discontinued. the electrostrictive ceramic stack is no longer polarized, and vibrations stop. In a third instance, a magnetostrictive stack is exposed to a direct current magnetic field via a coil and the stack material magnetic domains are aligned. An electrical signal applied to the coil causes the stack to generate vibrations.
One type of projector is a flextensional sonar projector, which is typically a low frequency transducer. Low frequency acoustic signals are desirable because they are less attenuated by the water through which they travel, which allows the signals to travel great distances. A flextensional transducer includes a transduction driver housed in a mechanical shell. The transduction driver is actuated by application of an electrical signal, which produces magnified vibrations in the shell thereby generating acoustic waves in the water. The shell vibrations are dependent upon the properties of the stack material included in the driver.
Flextensional acoustical projectors are used in active sonar applications, underwater seismic surveying, and other similar applications. Class VII and class IV flextensional projectors employ configurations which impart a substantial amount of stress on the transduction driver as the operating depth changes. For example, driver stress decreases with greater operating depth for class IV transducers. To provide sufficient stress at maximum depth, the driver must have a high initial stress. More specifically, the shell is used to pre-stress the driver by inserting the driver while the shell is under outward radial expansion. Relaxation of the shell places the driver in a compressed state. Structural limits associated with the high initial stress effectively limit the operating depth of the transducer.
Class VII transducers, on the other hand, have an opposite constraint, where operating stress increases with greater operating depth. This increase in stress reduces driver capabilities and performance with increased depth. In addition, conventional stress reduction techniques, such as delaying the application of stress to the driver, limit the shallow depth at which the transducer can operate.
What is needed, therefore, is a flextensional projector shell configuration having a stress profile that is substantially independent from depth of operation.
One embodiment of the present invention provides a flextensional transducer. The transducer includes a projector shell that is disposed about a transduction driver. The transduction driver has a first and a second end, and is adapted for receiving power from an alternating supply. The shell includes first and second bulbous end portions, each adapted to receive a respective end of the transduction driver. The shell further includes a middle portion that has both concave sections and convex sections, thereby defining a wave profile. In one particular embodiment, stress on the driver is substantially independent of operating depth. The flextensional transducer may further include a flexible water-proof material or boot covering the projector shell that is adapted to keep internal componentry (e.g., the transduction driver) dry.
Another embodiment of the present invention provides a flextensional transducer projector shell. The shell includes first and second bulbous end portions, and a middle portion having both concave sections and convex sections, thereby defining a wave profile. A recess may be defined in each respective bulbous end portion for retaining each end of a transduction driver. In one particular embodiment, the shell has a midpoint, and there are two opposing convex sections, each having a peak that is substantially aligned with the midpoint. In addition, there is a pair of opposing concave sections between each bulbous end portion and the opposing convex sections.
Another embodiment of the present invention provides a method of manufacturing a flextensional transducer projector shell. The method includes forming first and second bulbous end portions, and forming a middle portion having both concave sections and convex sections, thereby defining a wave profile. In one such embodiment, forming the first and second bulbous end portions includes forming a recess in each respective bulbous end portion for retaining each end of a transduction driver. The method may further include disposing the shell about a transduction driver with the driver""s ends each retained by a respective bulbous end portion. The driver can be adapted for receiving power from an alternating supply.
The features and advantages described herein are not all-inclusive and, in particular, many additional features and advantages will be apparent to one of ordinary skill in the art in view of the drawings, specification, and claims. Moreover, it should be noted that the language used in the specification has been principally selected for readability and instructional purposes, and not to limit the scope of the inventive subject matter.